For me you cannot beat two coats of two part epoxy primer, followed by the color coat of your choice, either single stage or base coat/clear coat. The epoxy bonds tight to clean steel, and the top coat is easily repairable. If you're going that way get all of your cutting, welding & grinding done, then hit it with primer before you use any type of filler, then once smooth & ready prime it again, or spot prime if your fill areas are small. Filler is best encased in primer, IMO.